The present invention relates to a doped optical fiber amplifier having an improved amplifier noise figure.
Doped optical fiber amplifiers consist of an optical fiber the core of which contains a dopant such as rare earth ions. Such an amplifier receives an optical signal of wavelength .lambda..sub.s and pump power of wavelength .lambda..sub.p which are combined by means such as one or more pump/signal wavelength division multiplexer (WDM) couplers located at one or both ends of the amplifier.
One of the key characteristics of a fiber optic amplifier is its noise figure, which is the ratio of the signal-to-noise ratio at the amplifier input and the signal-to-noise ratio at the amplifier output when the input noise is shot noise only. The noise figure characterizes the amount of noise the fiber amplifier adds to the signal. A signal is input to the amplifier and follows a primary signal path to the amplifier output. Fundamental unavoidable noise is generated in this direct path by spontaneous emission produced by the gain fiber and gives rise to a minimum noise figure. Excess noise is generated in the amplifier when a portion of the signal follows a second path and arrives at the output at some time delay relative to the signal in the primary path, thereby generating multi-path interference. One potential second path is a path formed by signals that experience two reflections somewhere inside the amplifier. In most amplifiers the largest source of reflections is the facets and coupling lenses of the pump lasers. Signal light reaches the pump laser facets due to the less than infinite isolation of the pump/signal WDM coupler. If the signal can be prevented from reaching the laser facet, the principal source of reflection within the amplifier can be removed, and the noise figure can be significantly reduced.
Multi-path interference also occurs in two stage amplifiers wherein residual pump power from the output of the first stage is coupled to the second stage by a pump path that is distinct from the signal path but which, due to poor isolation, propagates a small amount of signal. The signal component reaching the second stage via the pump path is delayed and out of phase with the direct signal and therefore converts the laser phase noise to intensity noise.